I remember watching a talk by someone who designed a telescope to view a single star at a time.

The purpose of the telescope was to detect waste-heat which would presumably be emitted by any civilization utilizing a large percentage of their planet's solar energy, due to the second law of thermodynamics. By focusing on a single star, it would be able to detect when the planet crossed the path of star-light, and the additional energy emitted by the planet - or something like this.

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This would be the SETI's Colossus telescope project, that aims to build a high-resolution, multiple-mirror instrument with ability to directly image the heat generated by other civilizations on planets orbiting stars near us:

As Earth-like civilizations evolve, they use more power. For example,
in Roman times, we estimate Ω [the ratio of a civilization’s power production to the amount of solar power it receives] was about 1/1000 what it is today.
Humans’ global power consumption is growing by about 2.5 percent per
year, even though the world’s population is growing at less than half
this rate. In contrast, our knowledge base (the combined total of all
recorded information) doubles in just two years. As cultures advance,
their information content also must grow, and the power required to
manipulate this knowledge eventually dominates a civilization’s total
power use.

Using a sensitive coronagraph to remove scattered light that would
obscure an exoplanet, Colossus would be able to find hundreds of
Earth-sized or larger planets in the habitable zone including any
civilizations on their surfaces.